When you buy a custom sapphire crystal, risk concentrates in one place: the gap between what you agreed verbally and what is written on the drawing. Put geometry, dimensional limits and finish on the drawing; put material identity, coating callouts and reference designations in the BOM; and put measured evidence, sampling method and sign-off conditions in the approval notes. Everything not assigned to one of these three layers is a dispute waiting for inspection day.
The Drawing–BOM–Gate framework
Most specification disputes in sapphire crystal sourcing are not caused by a bad supplier or a bad drawing. They are caused by a detail that exists in an email thread but in neither the drawing nor the approval record — so nobody can prove it was ever agreed.
The Drawing–BOM–Gate (DBG) framework removes that ambiguity by forcing every specification into one of three layers:
- Drawing — everything that defines the physical part: geometry, dimensional limits, edge shape, chamfers, print position, finish and the datum references a measurer needs.
- BOM — everything that identifies the part commercially and materially: material designation, coating type and which faces are coated, print/ink callouts, part number and revision, and the drawing revision the BOM line points to.
- Gate — everything that defines acceptance: which dimensions are measured and how, what instrument and method are used, the sampling rule for the lot, the reference samples both parties retain, and the named signatories who release production.
The discipline is exclusion as much as inclusion: if a detail is on the drawing, it does not need to live in an email; if it is in the approval notes, it does not need to be renegotiated at inspection.
Decision table: where each detail belongs
| Detail | Drawing | BOM | Approval notes | Notes for the buyer |
|---|---|---|---|---|
| Outer diameter, thickness and dimensional limits | Yes | Reference only | Measured evidence | State the measurement temperature and method reference where relevant. |
| Edge shape, chamfer, gasket-seat profile | Yes | No | Visual/section evidence | A section photo or profile sketch prevents fit disputes. |
| Flat, domed or curved form; radius callouts | Yes | No | Form measurement method | Define how form is verified, not just the number. |
| Surface finish and polish zones | Yes | No | Boundary-sample approval | Retain an agreed boundary sample for both sides. |
| Material designation (e.g., synthetic single-crystal sapphire, growth direction if specified) | Reference the drawing callout | Yes | Certificate of conformity if agreed | Ask the supplier what material documentation they can currently provide. |
| AR coating type, side, and number of layers | Callout symbol | Yes line item | Approved coated sample | Ask how coating conformity is demonstrated — written evidence, not assumption. |
| Print/logo: artwork, ink, position, colour | Dimensioned position on drawing | Ink and artwork reference in BOM | Approved print sample | Artwork revision must be version-controlled. |
| Date window or aperture | Yes | No | Position measurement record | Tolerance the window position relative to the stated datum, not free space. |
| Sampling rule and inspection method | No | No | Yes | Agree before release, not at shipment. |
| Reference/golden samples | No | Reference in BOM if numbered | Yes, with retention location and sign-off | Both parties keep samples under the same reference. |
| Rework and deviation rules | No | No | Yes | Define in advance how any deviation is requested and approved. |
The details buyers leave ambiguous — and the questions that close them
Three gaps appear repeatedly in custom crystal projects, and each is closable with a written question rather than a negotiation.
1. Datums and measurement references. A crystal's diameter can be measured at different heights of a chamfered edge; a domed crystal's height depends on where the apex is referenced. Ask: "Which datum features and measurement points does the supplier use, and can these be marked on the drawing before release?"
2. Coating and print conformity. "With AR coating" on a BOM line does not tell you which side, how many layers, or how conformity will be shown. Ask: "What evidence accompanies the coating — an approved coated sample, a written specification, or both — and is that evidence retained for future repair orders?"
3. Repair-part repeatability. If you will reorder this crystal for service movements in later years, the approval record is what makes the reorder auditable. Ask: "How long are the drawing, approval notes and reference samples retained, and what reference number ties a future reorder to this exact revision?"
Approval questions to put in writing before releasing the order
Use these as your release gate. A supplier's willingness to answer them in writing is itself information.
- Which dimensional limits on the drawing are measured on every piece, and which are sampled per lot?
- What measurement instrument and method will be used, and are the results documented per lot?
- Who signs the approval on each side, and in what form is the sign-off recorded?
- Are reference samples retained by both parties, under what reference number, and for how long?
- How are deviations or rework requests raised, documented and approved?
- What material documentation can be provided with the current order, and what can be repeated on reorders?
- What are the current MOQ, pricing, sampling policy and lead-time terms for this part — confirmed in writing for this project?
That last question matters because none of those commercial terms can be assumed. Sample policy, cost responsibility, shipping arrangements for samples, fees, lead time and capacity are all project-specific and subject to change; the only safe position is to confirm each one directly with the supplier at quotation stage and record the answer in the approval notes.
FAQ
Should tolerances go in the title block or next to each dimension? Wherever they go, they must be unambiguous and visible to whoever measures the part. General notes in a title block work only if every person reading the drawing also reads the notes. If a limit applies to a fit-critical feature, placing it at the feature removes interpretation risk.
Is a verbal agreement on a tolerance enough if the supplier confirms it by email? An email documents intent, but it is not the drawing. If a limit matters, it belongs on the drawing, and the evidence that the delivered parts meet it belongs in the approval notes. If it is not in either place, resolving a dispute becomes an argument about what was agreed.
What about AR coating — is it a drawing item or a BOM item? Both, with different roles. The drawing carries the callout symbol showing which surfaces are coated; the BOM carries the coating type as a purchasable line item; the approval notes carry the coated reference sample. Splitting it this way means artwork, geometry and finish each have one auditable home.
How do I make sure repair crystals ordered in future match this production run? Tie the reorder to three things: the drawing revision number, the retained reference sample number, and the approval record from the original run. Ask the supplier in writing how long they retain tooling records and samples for service parts — retention terms are project-specific and must be confirmed directly.
Do I need approval notes if the drawing is complete? Yes. The drawing defines the target; the approval notes define how you prove you hit it — measurement method, sampling rule, retained samples and signatories. Without the gate layer, "meets drawing" is an assertion rather than a record.
Put the framework to work
JadeZone Sapphire supports watch brands, component buyers and watchmakers in Shenzhen and Hong Kong with custom sapphire watch crystals and precision sapphire components. Verify your drawing package against the framework above, then bring your questions to us — we would rather answer them in writing before release than debate them at inspection.
[Download the drawing checklist →]
Prefer to start lighter? Use the short version: grab the one-page approval-question list and take it into your next supplier review.